To control the charging and discharging of a power storage device, it is necessary to accurately assess the dischargeable power and the chargeable power. In other words, it is necessary to accurately assess the open voltage (open-circuit voltage), the internal resistance, and the state of charge (SOC).
Directly measuring the state of charge of a power storage device is difficult. However, a degree of correlation between the SOC and the open voltage of a power storage device has been recognized. Accordingly, with the method of computing the SOC of a secondary battery for an electric vehicle disclosed in Patent Literature 1, the battery internal resistance of a secondary battery is computed by multiplying a predetermined resistance value, a first resistance ratio based on battery temperature, and a second resistance ratio based on a given reference state of charge. Subsequently, the open voltage is computed from the computed battery internal resistance as well as the current and voltage of a battery during charging or discharging, and the SOC of the battery is computed based on the correlation of the SOC with the open voltage.
The remaining battery capacity detection apparatus disclosed in Patent Literature 2 connects a load resistor to a secondary battery to cause a constant current discharge, and based on the voltage between the terminals immediately after starting the constant current discharge and after a certain time elapses, detects a polarization value dominated by internal mass movement or a resistance value dominated by internal mass movement based on how easily reactive matter inside the electrodes moves to a reaction site in the secondary battery. Subsequently, the SOC of the secondary battery is detected based on the polarization value dominated by internal mass movement or the resistance value dominated by internal mass movement.
The battery degradation measurement apparatus disclosed in Patent Literature 3 computes the internal resistance of a battery based on the battery voltages when different charging current values are supplied, and computes a battery cell degradation ratio based on the ratio against the internal resistance in an initial state.
The battery degradation level estimation apparatus disclosed in Patent Literature 4 uses relationship data obtained by pre-measuring the relationship between the charge amount and the open voltage value for each of different degradation levels, and computes a degradation level of a battery based on an electric charge of the battery computed by time-integrating a charge/discharge current value detected with a current sensor.